1. Field of the Invention
The present invention is related generally energy systems and methods for use in connection with large, off-highway vehicles such as locomotives and in particular, to a method and system for detecting a locked-axle condition in a vehicle operating in an isolated mode of operation.
2. Brief Description of the Prior Art
Locomotives and transit vehicles as well as other large traction vehicles are commonly powered by electric traction motors coupled in driving relationship to one or more axles of the vehicle. Locomotives and transit vehicles generally have at least four axle-wheel sets per vehicle with each axle-wheel set being connected via suitable gearing to the shaft of a separate electric motor commonly referred to as a traction motor. In the motoring mode of operation, the traction motors are supplied with electric current from a controllable source of electric power (e.g., an engine-driven traction alternator) and apply torque to the vehicle wheels which exert tangential force or tractive effort on the surface on which the vehicle is traveling (e.g., the parallel steel rails of a railroad track), thereby propelling the vehicle in a desired direction along the right of way. Alternatively, in an electrical braking mode of operation, the motors serve as axle-driven electrical generators, torque is applied to their shafts by their respectively associated axle-wheel sets, which then exert braking effort on the surface, thereby retarding or slowing the vehicle""s progress. The motor generated energy is dispersed in a grid of resistors.
Locomotives used for hauling applications are generally equipped with speed sensors coupled to respective traction motors or to the axles driven by the motors. The speed sensor data or information may be used to detect a locked axle condition. If a locked axle condition occurs on a given axle while the locomotive is moving, the rotational speed of that axle decreases to zero, and all other axles rotate at a speed corresponding to the speed of the locomotive. Thus, a locked axle condition can be detected whenever a substantial difference in speed is sensed in one axle relative to the other axles.
In addition, when a speed sensor fails to supply reliable sensor data, a locked axle condition may be indicated in a system controller since existing controllers are not generally capable of detecting speed sensor failures. When a locked axle condition is indicated, the operator generally has to stop the train and visually inspect the axle or wheels for any discernable damage. If the visual inspection does not reveal any useful information, then the train may have to be rolled slightly to ascertain whether in fact the axle is locked or whether the condition was indicated due to a faulty speed sensor. If the latter is true, then the locomotive can be operated at lower speeds until the speed sensor and/or speed detection system can be repaired. Stopping trains, inspecting axles, and operating at lower speeds involve burdensome delays.
One method of monitoring for locked axle conditions is generally achieved using speed sensors on the traction motors. However, the speed sensor systems have high failure rates due to the treacherous environment in which they operate. Co-assigned U.S. Pat. No. 5,990,648 entitled xe2x80x9cMethod for Detecting Locked-Axle Conditions Without a Speed Sensorxe2x80x9d issued to Kumar et al., describes a method of detecting locked axle conditions without the use of speed sensors.
However, situations involving heavy haul applications generally use multiple locomotives in a consist to provide the total power/tractive effort required to run the train from the starting point to the destination point. Invariably the number of locomotives in the consist may be larger than the number of locomotives required for several portions of the trip. Diesel-electric locomotives are often loud and the vibrations associated with the engine make the environment uncomfortable for train operators. Therefore, operators tend to isolate one or more locomotive units in the consist to save fuel and/or reduce the noise and vibration in the units for crew comfort. The diesel engine in the isolated locomotive is run at low speeds and the alternator field and its control are disabled. In such a condition, the diesel engine cannot be used as a source of electrical power.
The method disclosed in the ""648 patent requires the traction inverter to control the traction motor so as to produce flux in the traction motors so as to produce appropriate polarity torque to determine if the motors are turning or not. Therefore these traction inverters need to have DC bus voltage to achieve this function. However, when a locomotive is put into isolate mode, the DC bus cannot be powered by the diesel engine and the locked axle detection without speed sensors method as disclosed in the ""648 patent can not be used. This causes many locomotives to be unnecessarily shopped for speed sensor failure. Thus, there is a need for a method which will reliably distinguish between an actual locked axle condition and an erroneously detected axle condition indicated based on a speed sensor failure for use with a locomotive operating in an isolated mode.
In one embodiment, the invention is a method for detecting the occurrence of an actual locked condition in one or more of a plurality of AC traction motors which are reconfigurable to operate as AC generators, wherein said AC traction motors connected to a common DC bus. The method includes detecting a potential locked condition in a first motor, energizing the DC bus with an initial voltage using an alternate source of power, and regulating the DC bus voltage with a second motor of the plurality of AC traction motors by reconfiguring the second motor to operate as a generator. The method further includes measuring torque produced by the first motor at a plurality of levels of electromagnetic flux in the first motor, and determining based on the measured torque whether the potential locked condition is an actual locked condition.
In another embodiment, the invention is a system for detecting the occurrence of an actual locked-axle condition in an isolated vehicle having a plurality of AC traction motors which are reconfigurable to operate as AC generators, said AC traction motors connected to a common DC bus. The system includes a speed sensor detecting a potential locked axle condition in an axle coupled to a first motor of the plurality of AC traction motors, and a power supply energizing the DC bus with an initial voltage using an alternate source of power. The system further includes a voltage regulator regulating DC bus voltage with a second motor of the plurality of AC traction motors by reconfiguring the second motor to operate as a generator, and a torque sensor measuring a torque produced by the first motor at a plurality of levels of electromagnetic flux in the first motor. The system further includes a processor determining based on the measured torque whether the potential locked-axle condition is an actual locked axle condition.